Process metrology includes a plurality of control and regulation tasks. A particular challenge faced is the regulation of processes that have such a dead time profile between a system input and a system output that a change in at least one parameter of the first type at the system input of the process causes a change in at least one parameter of the second type of the process medium at the system output only after a dead time elapses.
Typical examples of such processes are wastewater treatment processes in wastewater treatment plants. This is illustrated by the example of the phosphate elimination below.
During the addition of precipitant for precipitation of phosphate from wastewater to be treated manually or by means of metering pumps in manual mode, overdosing or underdosing occurs time and again. Underdosage of precipitant leads to increased phosphate concentrations at the wastewater treatment plant outlet. In contrast, overdosage leads to increased formation of hydroxide sludge, whose disposal causes time and expense, and which takes up space in the digester to the disadvantage of the biomass that is beneficial for the biogas. Therefore, it is desirable to avoid over and underdosages of precipitant during phosphate elimination.
Methods of phosphate elimination, in which a simple time control that considers a typical diurnal variation of the phosphate load is applied are known in the prior art. Methods of phosphate elimination, which comprise load-dependent control system of precipitant dosage are also known from the prior art. Such load-dependent control determines the current influx, for example by means of a measurement of the volumetric flow rate, and the phosphate concentration and calculates the precipitant needed for phosphate elimination on the basis of a proportionality factor, for example in the form of a precipitant mass or volumetric flow rate, which must be added using a metering pump, for instance to wastewater flowing into the aeration tank. Besides a safety factor, this proportionality factor essentially includes the product properties of the precipitant.
However, it has been observed that a number of potential sources of error cannot be sufficiently compensated by such controls. For example, the composition of the precipitant varies from batch to batch or the concentration of the precipitant solutions prepared on-site can fluctuate because of errors or inaccuracies in the preparation of the solutions. Changes in the wastewater matrix, i.e. the composition of the phosphate-containing wastewater can also have an impact on the precipitant quantity necessary for the precipitation of a particular phosphate concentration in the wastewater. Activated sludge, which is needed in the biological stage of a wastewater treatment plant can also store phosphate and set free later (simultaneous precipitation), which can occur in varying degrees with changes in the operating conditions. The most serious drawback of the load-dependent control systems known from the prior art is that they do not include monitoring of the effectiveness of the precipitant dosage, i.e., changes in process conditions, for example due to the mentioned sources of error are not recognized by the control system, i.e. not “noticed”, and are therefore, not compensable by the control system.
Another way to optimize the precipitant dosage is by regulation of the degraded phosphate quantity with the metered precipitant quantity as an adjustable variable. Such regulations are often configured so that the volumetric flow rate at the inlet to a feed point for the precipitant, which binds ortho-phosphate by a precipitation reaction, is applied as a disturbance variable and the regulator regulates the phosphate concentration at the outlet to a predetermined target value. However, the dead time profile of the process is problematic here.
The dead time profile is based on the fact that in most wastewater treatment plants, the wastewater to be treated requires a certain flow time from the point of precipitant addition up to the measuring point, at which a control variable that can be used for a regulator can be detected. This flow time specifically depends on the local conditions and the type of the precipitation method applied in each case (pre-precipitation, simultaneous precipitation, post-precipitation). Typically, the measuring point at the outlet, where the phosphate concentration is detected, is far from the inlet and from the metering point, where the precipitant is added to the wastewater. The flow time of the wastewater between the inlet and outlet measurement (outlet of the wastewater treatment plant, or between biological level and post-clarification) is usually several hours. Load changes are noticed only at the measuring point in the outlet and therefore, can no longer be compensated for by an increase in the precipitant. In the case of sudden changes in the load, such a regulation responds with delay, so that volumes of wastewater to be treated cannot be avoided in the case of over and underdosing. The large flow times that affect the dead time in the regulated path, also lead to the fact that in case of a variation of the feed stream, the volumetric flow rate that is currently measured at the inlet does not correspond to that at the measuring point and/or the metering point.
Similar problems may also occur in processes other than phosphate elimination, in particular, processes of wastewater treatment or treatment of other process media with a dead time profile due to high flow times.